The present invention relates generally to centrally-obscured reflective telescopes, and more particularly, to such as a centrally-obscured reflective telescope having a semi-active focus and thermal compensation system that also may be used for boresight correction (line-of-sight control).
A centrally-obscured reflective telescope, such as a three mirror anastigmat telescope, for example, includes a primary mirror, a secondary mirror, and a tertiary mirror. The telescope is generally paired with focusing optics or an imager that focuses an image produced by the telescope onto a detector. The telescope may be protected by a window located in object space. The secondary mirror is suspended between the window and the primary mirror by means of a plurality of support struts. The window is exposed to external temperatures and there is a temperature gradient between the exterior of the window and the primary mirror.
The three mirror anastigmat telescope provides optimum performance only within a narrow isothermal range. Many applications require the secondary mirror to be located near the window where external temperatures are extreme (high altitude in aircraft, for example), and the cavity temperature must be controlled, resulting in a large thermal gradient near the window. To minimize system and window size, the secondary mirror is generally located close to the window, and is thus within this gradient area. Techniques used to heat the window or otherwise minimize the temperature gradient take substantial power, cause gimbal disturbances due to airflow, and the like. Furthermore, these techniques do not eliminate asymmetric temperatures if the relative angle of the telescope with respect to the window is not constant (gimbal pointing). This will cause boresight shift.
The problem is that the telescope must be maintained isothermal in order to remain in focus. In particular, in order for the telescope to remain in focus, the relative curvatures of the primary and secondary mirrors must be maintained as well as the separation between the primary and secondary mirrors. Due to a difficult thermal environment and local power dissipation, this has not been possible using external cavity temperature control.
One prior approach to compensate for temperature variations involved the use of a mechanical drive system that displaces the focal plane. This approach is difficult to align initially, and is susceptible to shifts. The primary issue is that movement of the focal plane, or primary and secondary mirrors along the optical axis by as little as fifty-millionths of an inch will result in defocusing of the telescope. Actuator drives, position sensors and servo controls are required. Mechanical elements are degraded by dirt, temperature variations, shock and vibration, corrosion, lubricant degradation, calibration drift with temperature, hysteresis and friction. Furthermore, there may be optical prescription errors, and the like, that cannot be corrected by such mechanical drive system. These effects make it almost impossible to control the relative positions of the focal plane, primary and secondary mirrors with sufficient accuracy to focus the telescope.
It would therefore be an advantage to have an improved centrally-obscured reflective telescope that embodies a focus and thermal compensation that improves upon conventional approaches. Accordingly, it is an objective of the present invention to provide for an improved centrally-obscured reflective telescope having a semi-active focus and thermal compensation system. It is a further objective of the present invention to provide for an improved centrally-obscured reflective telescope having a semi-active focus and thermal compensation system that may be used for boresight correction (line-of-sight control).
To meet the above and other objectives, the present invention provides for a centrally-obscured reflective telescope, such as a three mirror anastigmat telescope, for example, that employs a semi-active focus and thermal compensation system. An exemplary three mirror anastigmat telescope comprises an insulated housing having a cavity and an input window. The telescope also comprises a primary mirror, a secondary mirror, and a tertiary mirror. The tertiary mirror is not required for all embodiments of the centrally-obscured reflective telescope. The secondary mirror is suspended between the input window and the primary mirror by means of a plurality of support struts. A cylindrical housing or barrel has the support struts secured to one end thereof and is secured to the primary mirror at the other end. The telescope is generally mated to focussing optics, such as an imager. A fold mirror may be used to reflect light from the secondary mirror onto the tertiary mirror which couples the light to the focussing optics or imager. A cavity heater may be provided in the cavity for heating the interior of the cavity.
The semi-active focus and thermal compensation system comprises a temperature compensation controller that is coupled to heating elements disposed on various components of the telescope. The temperature compensation controller is coupled to a temperature sensor disposed on each of the support struts, to a temperature sensor disposed on the secondary mirror, to a temperature sensor on disposed the primary mirror, and to a plurality of temperature sensors disposed around the cylindrical housing or barrel. The temperature sensors may comprise thermistors, for example. In general, it is desirable to have the temperature sensor on each support strut aligned with temperature sensors on the cylindrical housing or barrel.
As was mentioned in the Background section, to keep the telescope in focus, the relative curvatures of the primary and secondary mirrors must be maintained and the separation between the primary and secondary mirrors must be held constant. The semi-active focus and thermal compensation system senses temperatures of the support struts, the barrel, and the primary and secondary mirrors using the temperature sensors. The primary mirror is used as a reference, although another component may be selected.
The barrel and the support struts are selectively heated by the heating elements under control of the temperature compensation controller to control their relative lengths which in turn controls the position of the secondary mirror relative to the primary mirror and hence the focus and boresight of the telescope using servo-type balance. The secondary mirror is selectively heated to control its curvature relative to the primary mirror. These temperature control actions maintain the relative curvatures of the primary and secondary mirrors and the separation between the primary and secondary mirrors relatively constant in order to keep the telescope in focus.
The heating elements are tailored to expected gradients and are shielded from the optical path of the telescope. The temperature compensation controller provides closed loop feedback control of the strut, barrel and mirror temperatures, and thus, focus and thermal characteristics of the telescope. The temperature compensation controller may control the heating elements in an analog (hardwired) fashion, or the heaters may be software controlled, depending upon requirements.
The heating elements attached to the support struts, barrel and secondary mirror athermalize the telescope, provide focus control, and mitigate boresight drift. This is accomplished by multipoint temperature sensing and proportional heater power control using the temperature compensation controller. The semi-active focus and thermal compensation system may be operated in a balance mode, providing only athermalization, or active boresight correction and focus control may be provided by the controller.